Russian Deputy Foreign Minister Sergei Ryabkov said: "We are pleased that our persistent calls for the return of UN experts to Syria for the investigation of other episodes have borne fruit."

Mr Ryabkov was speaking in parliament, referring to alleged incidents of chemical weapons use other than the attack near Damascus on 21 August.

He also said talks with the US on Syria are going "not so smoothly" and that US officials "always mention" that plans to punish Damascus remain in force.

"We draw certain conclusions from that and assume that the threat of aggression in violation of international law is so far only delayed, not dismissed fully," he said.

A timetable for President Bashar al-Assad to hand over Syria's chemical weapons has been laid down by US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov.

Security experts say Syria has about 1,000 tonnes of mustard gas, VX and sarin, which is the nerve agent UN inspectors found after hundreds were killed by poison following missile strikes on rebel-held areas last month.